An investigation of new barium phosphate chemical conversion coating on AZ31 magnesium alloy

Barium phosphate chemical conversion coating is a new research area for the corrosion protection of Mg alloys. We report a new process conducted in a simple solution with Ba(NO3)2 and NH4H2PO4 as the precursors. The influences of pH value and temperature of the coating bath, concentrations of Ba(NO3)2 and NH4H2PO4, and deposition time were evaluated. The coating obtained typically featured a two-layer structure. The chemical compositions and structures were then further characterized with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), revealing an under layer of mixed amorphous phosphates and a top layer of crystalline BaHPO4. The corrosion resistances of bare Mg alloy and those with coatings were compared with electrochemical polarization, showing an improved corrosion resistance of samples with coating. More specifically, the two-layer coating was shown to be more effective than with the under layer coating alone. In addition, adhesion test was also conducted to evaluate the bonding strength and to identify the separation interface of the coatings.